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Bones

Locomotion is generally brought about by a system of muscles in conjugation with a skeleton. The skeleton may be an endoskeleton, an exoskeleton or a hydrostatic skeleton. The support system will be adapted to methods of locomotion for a particular animal (e.g, flying, swimming, climbing, and walking).

The central portion of a bone contains marrow. This may be yellow (a fat store) or red (the site of red blood cell and some white cell production). The marrow is surrounded by compact bone that withstands forces in a downward direction.

The head of the bone may have spongy bone between the compact bone and the marrow. Spongy bone also runs down the length of most bones other than the long bones of the arms and legs. This can withstand forces from many directions. This arrangement allows bones to be strong yet light.

Bone is also made up of a matrix and lacunae inhabited by cells, osteocytes, which secrete the matrix.

It has many blood vessels through it, as the rate of diffusion would be too low to keep the cells alive. The matrix is like that of cartilage but it contains a complex calcium phosphate mineral that makes the bone hard and strong. It is prevented from being brittle by collagen.

Between the lacunae are small channels (canaliculi). These contain more osteocytes. Capillaries and nerves run through other channels called Haversian canals.

Other cells, osteoclasts, dissolve bone with acid that they produce, and then osteocytes lay down new bone. This allows the bone to change shape if necessary and it keeps the bone tissue relatively young.